Design, Synthesis and Catalytic Activity of Di-N-Heterocyclic Carbene Complexes of Nickel and Palladium

Abstract

N-heterocyclic carbenes (NHC) have widely been used as spectator ligands in organometallic chemistry. Chelating bidentate di-N-heterocyclic carbenes (diNHC) provide additional entropic stability to their complexes relative to monodentate analogues. The steric and electronic environment around the metal centre can be fine-tuned by varying the substituents on the nitrogen atoms of the diNHC ligand. Synthesis and characterization of air and moisture stable bis(diimidazolylidene)nickel(II) complexes, [(diNHC)2Ni]2+, and their corresponding silver(I) and palladium(II) analogues are described.

Investigations into the catalytic potential of diNHC complexes of nickel as an alternative to palladium systems in carbon-carbon coupling reactions are discussed. In the Suzuki-Miyaura coupling reaction, the [(diNHC)2Ni]2+ complex was active for the coupling of aryl chlorides as well as aryl fluorides. The analogously synthesized Pd(II) complexes resulted in formation of (diNHC)PdCl2 species which were not active for the coupling of aryl fluorides. “Transition-metal free” coupling reactions were investigated and the results indicated that in the Mizoroki-Heck reaction, aryl iodides could be activated in the absence of nickel or palladium precatalysts when using Na2CO3 or NEt3 as base, while in the Suzuki-Miyaura reaction, aryl iodides and aryl bromides could be activated without any precatalyst when K3PO4 was used as base.

A general route into the synthesis of non-symmetrically substituted ligand precursors has been developed. Synthesis and characterization of non-symmetrically substituted ligand precursors, and their corresponding silver(I), palladium(II) and nickel(II) complexes are described. The activity of one of the non-symmetrically substituted (diNHC)Pd(II) complexes in the Suzuki-Miyaura coupling reaction of bulky substrates has been investigated. Non-symmetrically substituted diNHC ligand precursors with a hemi-labile pyridine arm have been synthesized and their corresponding Ni(II) and Pd(II) complexes are described.

Attempts to synthesize three-coordinate Pd(II) complexes using bulky â-diketiminato ligands are also discussed.